This invention relates to a method of producing ternary shape-memory alloy films by sputtering process techniques. In particular, it relates to a method of producing nickel-titanium-hafnium shape-memory alloy films by sputtering process techniques using krypton as a process gas.
Various metallic materials capable of exhibiting shape-memory characteristics are well known in the art. These shape-memory capabilities occur as the result of the metallic alloy undergoing a reversible crystalline phase transformation from one crystalline state to another crystalline state with a change in temperature and/or external stress. In particular, it was discovered that alloys of nickel and titanium exhibited these remarkable properties of being able to undergo energetic crystalline phase changes at ambient temperatures, thus giving them a shape-memory. These alloys, if plastically deformed while cool, will revert, exerting considerable force, to their original, undeformed shape when warmed. These energetic phase transformation properties render articles made from these alloys highly useful in a variety of applications. An article made of an alloy having shape memory properties can be deformed at a low temperature from its original configuration, but the article xe2x80x9cremembersxe2x80x9d its original shape, and returns to that shape when heated.
For example, in nickel-titanium alloys possessing shape-memory characteristics, the alloy undergoes a reversible transformation from an austenitic state to a martensitic state with a change in temperature. This transformation is often referred to as a thermoelastic martensitic transformation. The reversible transformation of the NiTi alloy between the austenite to the martensite phases occurs over two different temperature ranges which are characteristic of the specific alloy. As the alloy cools, it reaches a temperature (Ms) at which the martensite phase starts to form, and finishes the transformation at a still lower temperature (Mf). Upon reheating, it reaches a temperature (As) at which austenite begins to reform and then a temperature (Af) at which the change back to austenite is complete. In the martensitic state, the alloy can be easily deformed. When sufficient heat is applied to the deformed alloy, it reverts back to the austenitic state, and returns to its original configuration.
Shape-memory materials previously have been produced in bulk form, in the shape of wires, rods, and plates, for utilities such as pipe couplings, electrical connectors, switches, and actuators, and the like. Actuators previously have been developed, incorporating shape-memory alloys or materials, which operate on the principal of deforming the shape-memory alloy while it is below its phase transformation temperature range and then heating it to above its transformation temperature range to recover all or part of the deformation, and, in the process of doing so, create moments of one or more mechanical elements. These actuators utilize one or more shape-memory elements produced in bulk form, and, therefore are limited in size and usefulness.
The unique properties of shape-memory alloys further have been adapted to applications such as micro-actuators by means of thin film technology. Micro-actuators are desirable for such utilities as opening and closing valves, activating switches, and generally providing motion for micro-mechanical devices. It is reported that the advantageous performance of micro-actuators is attributed to the fact that the shape-memory effect of the stress and strain can produce substantial work per unit of volume. For example, the work output of nickel-titanium shape-memory alloy is of the order of 1 joule per gram per cycle. A shape-memory film micro-actuator measuring one square millimeter and ten microns thick is estimated to exert about 64 microjoules of work per cycle.
The most well known and most readily available shape-memory alloy is an alloy of nickel and titanium. With a temperature change of as little as about 10xc2x0 C., this alloy can exert a force of as much as 415 MPa when applied against a resistance to changing its shape from its deformation state.
Although numerous potential applications for shape-memory alloys now require materials featuring phase transformation temperatures above about 100xc2x0 C., the martensite start point for the common commercially available nickel-titanium alloys barely exceeds about 80xc2x0 C. In order to meet higher temperature applications, ternary alloys have been investigated, using various additional metallic elements. For example, substitution of noble metals (Au, Pd, Pt) for Ni in NiTi alloys successfully accomplishes higher temperature phase transformations, but the costs introduced are somewhat prohibitive for many commercial applications. Ternary nickel-titanium base shape-memory alloys including a zirconium or hafnium component appear to be potentially economical high temperature transformation candidates. However, particularly in either Ti(NiPd, Pt) or Ni(TiHf, Zr) systems, there exists a challenge to develop a reliable process for producing microns-thick, thin films of these high temperature shape-memory alloys.
Now, an improved method of fabricating ternary shape-memory alloys using sputtering techniques has been developed.
According to the present invention, there is provided a method for producing a thin film deposit of a ternary alloy exhibiting mechanical shape-memory characteristics by using a sputtering deposition process comprising a sputtering deposition process wherein krypton serves as a process gas.
Previously practiced sputtering deposition processes for fabricating thin films of binary shape-memory alloys, such as nickel-titanium alloys, have utilized argon as the process gas during the sputtering deposition process. Before introduction of the argon process gas as the ionizing medium during sputtering in a sputtering chamber, the chamber first is evacuated in order to avoid introduction of oxygen contamination during the process, which would adversely impact the properties of the deposited film. Oxygen has been shown to decrease transition temperatures and adversely affect the mechanical properties of the film.
Likewise, in ternary shape-memory alloy film deposition, argon typically has been employed as the process gas during sputtering. However, production of ternary shape-memory alloy thin films by sputtering techniques using argon as the process ionizing gas results in weak, brittle films that do not meet micro-actuator grade requirements. These inferior properties were not unexpected, since bulk ternary shape-memory alloys also tend to exhibit mechanical properties far inferior to bulk binary shape-memory alloys. Surprisingly, it now has been discovered that the use of krypton as a process gas significantly enhances the quality of a ternary shape-memory alloy thin film applied by sputtering deposition.